Week 12 - Excitable Tissues Flashcards
What are leak channels
Pores in the cell membrane which is open all of the time
What are gated ion channels
Pores/transmembrane proteins in the cell membrane which is only open when the cell is stimulated. When they are not activated they are closed and ions cannot travel through
What is intracellular fluid
the fluid inside the cell, which is mainly a salt solution (ion solution)
What is extracellular fluid
the fluid which surrounds the cell which is also mainly a salt solution (ion solution)
What are the main ions which form action potential
sodium (Na+) and potassium (K+)
What is the proportion of Na+ and K+ ions inside and outside of the cell
there is a higher concentration of Na+ in the extracellular fluid and a higher concentration of K+ in the intracellular fluid
What is resting membrane potential
Resting Membrane potential is the electrical potential difference across the cell membrane of excitable cells when they are not actively transmitting signals. The potential difference exists because the inside of the cell is negatively charged relative to the outside.
How is resting membrane potential formed in excitable cells
- K+ ions have a high permeability through leak channels. Since there is a high concentration of K+ inside the cell in comparison to outside, K+ diffuses out of the cell along its concentration gradient. This makes inside the cell more negatively charged and outside
- The cell’s “battery” is now charged
- When the 2 forces of positive and negative charges are balanced and there is no more net movement of K+ it is referred to as the equilibrium potential
- However whilst potassium diffuses until its concentration it starts to get “pulled back” by the negative charges on the inside of the cell membrane as K+ is positively charged
In the formation of resting membrane potential what is the movement of K+ influenced by
- The concentration gradient is still driving K+ to leave the cell
- Electrical gradient that is pulling K+ back into the cell
What is action potential
A regenerating depolarization of membrane potential that propagates along an excitable membrane
What are the factors which allow for the generation of action potential (2)
- A resting membrane potential (polarized capacitor)
- The cell membrane can briefly become permeable to Na+ ions when the nerve is stimulated
How is action potential created
- Resting Stage
- This is the resting membrane potential before the action potential begins - Depolarization Stage
- When the cell becomes stimulated Na+ ion channels open, and Na+ ions flow down its concentration gradient into the cell (also due to the electrical gradient)
- This causes the cell to become increasingly more positively charged, and to the extent that it becomes more positively charged than outside the cell
- This is the action potential - Repolarization Stage
- Shortly after the formation of the action potential the Na+ ion channels close and the cell membrane is impermeable to Na+
- The K+ ion channels open and K+ ions pass out of the cell which re-establishes the normal negative resting membrane potential
- As a result of the action potential the cell has gained some Na+ ions and lost some K+ ions. In oder to get things back to normal a sodium potassium pump (N+/K+ ATPase) uses ATP (active transporter) pumps Na+ ions out of the cell and K+ ions into the cell in order to restore normal concentrations
What are the 3 connective tissue sheath of skeletal muscle
- Epimysium
- Perimysium
- Endomysium
What is epimysium
(most superficial) An overcoat of dense irregular connective tissue that surrounds the entire muscle. This tissue protects the muscle and reduces friction during contraction.
What is perimysium
Fibrous connective tissue that surrounds groups of muscle fibres called fascicles
What is endomysium
(deepest) Fine sheath of connective tissue composed of areolar and reticular fibres surrounding each muscle fiber
What do the 3 connective tissue sheaths of muscle form
Tendons. Tendons transmits forces across joints or bones.
What is sarcolemma
The plasma membrane of the muscle cell which is the outer covering of a muscle, enclosing the sarcoplasm (cell’s cytoplasm). Multiple nuclei lie just under the sarcolemma.
What are t-tubules
Specialized structures which penetrate deep into the interior of the muscle cell. It transmits action potential from the cell surface to the interior of the muscle fiber, allowing for muscle contraction.
What is sarcoplasm
The cytoplasm of muscle cells. It’s the cellular material that surrounds the myofibrils, which are responsible for muscle contraction.
What are myofibrils
- Myofibrils are the contractile elements of the cell which are densely packed, rod-like, running parallel to the length of the fiber
- Mitochondria and other organelles are squeezed in between them
- They make up 80% of the cell
- Myofibrils within a fibre are perfectly aligned and have a series of repeating dark (a) bands and light (I) bands
What are sarcomers
- The repeating segments of the myofibrils. The Sarcomere is the region of a myofibril between 2 successive Z discs
- Sarcomeres consist of a dark A band in the middle and half light I bands at each end
- Z discs are sheets of proteins (connectins) which anchor the muscle filaments and connects myofibrils to one another
What are myofilaments
the protein filaments that make up the structural components of myofibrils, the contractile units of muscle cells (thick and thin filaments)
What are the main types of myofilaments (3)
- Thick filaments
- Thin filaments
- Elastic filaments
What are thick filaments
- Composed of the protein myosin
- Located in the centre of the sarcomere
- They run the entire length of the dark A band
What are thin filaments
- Composed of the protein actin
- Think filaments are anchored to the Z disk
- They extend across the light I band and part way into the dark A band
What are elastic filaments
- Composed of protein titin
- they extend from the Z disc to the thick filaments, then run within the thick filaments
- They help hold the thick filaments in place, but they also help the muscle spring back into shape after it has been stretched or has contracted
What is the structure of thick filaments
- Comprised primarily of protein myosin
- Each myosin molecule has a rod like tail and 2 globular heads
- The tails are attached via a hinge to the heads and are able to attach and form cross bridges with the thin filaments (this requires ATP)
What is the structure of thin filaments
- Comprised of protein actin
- They are actin subunits which form a long helical chain composed of 2 strands
- The globular actin subunits contain active sites which myosin heads attach to during contraction however there are regulatory proteins tropomyosin and troponin which covers/blocks the active sites on the actin
What is the H zone
only thick filament and no thin filament causing it to have a lighter image
What is A band
Where the thick filaments are found
What is I Band
where the thick filaments are not
What is the sarcoplasmic reticulum
- Comprised of terminal cisternae and longitudinal tubules
- terminal cisternae form junctional feet adjacent to the t-tubule membrane
- It’s an intracellular storage compartment for Ca2+
What is the triad
The combination of the 3 structures t-tubules, sarcoplasmic reticulum (terminal cisternae and longitudinal tubules) is referred to as the “Triad”
What is a motor unit
the single motor nerve and all the muscle fibers it supplies
What are the 3 phases to a muscle twitch
- Lag Period - Time between stimulation and excitation-contraction coupling
- Period of Contraction - Cross bridges form, tension increases and muscle may shorten
- Period of Relaxation - Ca2+ re-absorbed and muscle tension returns to zero
How does the frequency of stimulation impact degree of muscle contraction
If there are very frequent stimuli the muscle doesn’t have time to completely relax resulting in wave summation (the waves superimpose). If stimuli are given quickly enough it results in tetanus, which is continuous contractions.
How does the strength of stimulus impact the degree of muscle contraction
The strength of the stimulus is proportionate to the force of the contraction. If the strength of the stimulus increases so does the force of the contraction once over the threshold stimulus.
The force of contraction increased through the phenomenon called recruitment, which means that more muscle fibers are brought into play.
What is the threshold stimulus
stimulus strength at which the first observable muscle contraction occurs
How does action potential propagate down a bare plasma membrane
voltage of the action potential will decay quickly and will not be able to propagate down the dendrite as the current leaks across the membrane
How does action potential propagate down a un-methylated axon
axon which contains voltage gated Na+ and K+ channels but is unmyelinated, it can regenerate the action potential at each point there is an ion channel, so that the voltage doesn’t decay. However the conduction of the action potential is slow because of the movements of ions and of the gates.
How does action potential propagate
- At a small area of the axon there is an Na+ influx causing the axonal membrane to depolarize, and the positive ions in the nerve axoplasm move towards the negative portion of the membrane
- Similarly, positive ions in the extracellular move towards the newly negatively charged area in the excellular fluid.
- A current is created that depolarizes the adjacent membrane in a forward direction. This impulse propagates away from its point of origin.
What is the process of synaptic signaling (formation of stimulus from nerves)
- Neurotransmitter synthesis
- Synaptic vesicles are formed from budding Golgi and are transported to the terminal
- Acetylcholine (ACh) is the neurotransmitter and is formed in the cytoplasm and is transported into the vesicles - Action Potential Propagation
- (1)Action potential begins in the ventral horn of the spinal cord and it depolarizes the axon membrane and opens voltage gated Ca2+ channels - Neurotransmitter Release
- (2)The influx of Ca2+ ions trigger the fusion of the synaptic vesicles with the pre-synaptic membrane (end of the axon) and ACh (neurotransmitters) are released into the synaptic cleft (space in between the end of the axon and the skeletal muscle fiber) via exocytosis - Neurotransmitter Binding
- (3)Released ACh diffuse across the synaptic cleft and binds to specific receptors which are typically ligand-gated ion channels or G protein couple receptors.
- This can be a stimulus for the formation of action potential in skeletal muscle
